TUESDAY, Sept. 8, 2015 (HealthDay News) -- People with a family history of obesity who believe their genes doom them to the same may "give up" and eat worse, a new study suggests.

The study found that when it comes to weight, feelings of powerlessness against one's DNA was tied to a higher body mass index -- a measurement that takes into account weight and height.

A person with such an outlook "may engage in more behaviors that are rewarding in the short term, such as eating unhealthful foods and avoiding exercise," wrote study co-authors Drs. Jessica Alquist and Mike Parent. People who believe that genes dictate body weight may also avoid "healthful behaviors with more long-term benefits for weight management," the researchers said.

However, if doctors fight the notion that a patient's weight is "unchangeable," they might help a person's motivation to eat better and exercise more, the investigators said.

One expert in nutrition and obesity agreed.

"We are predisposed to our genetic makeup, but we can still control our environment, physical activity -- anything is better than nothing -- and, like everything else in life, a change in one area will necessitate change in other areas," said Sharon Zarabi, a nutritionist at Lenox Hill Hospital in New York City.

In their study, Alquist and Parent looked at data from almost 4,200 men and nearly 4,700 women. The study found that as people get older, the idea that weight is determined more by DNA than factors people can control -- such as diet and exercise -- is associated with poor eating habits. These included markers for poor diets such as eating fewer fruits and vegetables, dining out a lot, eating lots of frozen or ready-to-eat meals (such as frozen pizzas), and paying little attention to nutrition labels on food products.

Men and women who believed their weight was genetically determined and out of their control were also less likely to exercise, according to the study.

However, according to Zarabi, people need to understand that genes are not necessarily destiny, including when it comes to body weight.

While giving "excuses" for overeating is common, "when we assume responsibility for making changes -- whatever the sacrifice may be -- we are investing a part of ourselves in the planning process," she said. "This is what will ultimately increase our motivation to achieve long-term success."